1. Field of the Invention
The present invention relates to a screw apparatus, in which a self-tapping screw is screwed to a case by screwing the self-tapping screw into a through hole formed at the case. The present invention relates, more particularly, to a system, in which an internal mechanism (e.g., a gear mechanism) is provided inside the case.
2. Description of Related Art
There have been utilized various screw apparatuses, in which a screw having a male thread portion is screwed into a case. As one example, a valve drive assembly of an electronic throttle will be described to explain the conventional art (see, for example, Japanese Unexamined Patent Publication No. 2000-110589 corresponding to U.S. Pat. No. 6,694,947 and Japanese Unexamined Patent Publication No. 2002-256894 corresponding to U.S. Pat. No. 6,834,639).
A housing case 104 for a drive system of the electronic throttle includes an electric drive device and a middle opening position returning device (middle position locating device) as shown in FIG. 6. Here, the electric drive device changes an opening position of a throttle valve (not shown) using electric power. The middle opening position returning device stops the throttle valve at a middle opening position.
The electric drive device includes an electric motor 107 and a gear mechanism 108. Here, the electric motor 107 is electrically controlled by an engine control unit (ECU), which is not shown, and the gear mechanism 108 reduces a rotational output of the electric motor 107 and delivers the output to a throttle shaft, which is rotatable integrally with the throttle valve (not shown).
The middle opening position returning device returns (locates) a stop position of the throttle shaft back to a middle position using a restoring force of springs when the electric motor 107 is stopped. Here, the middle position corresponds to a middle stop opening degree for the throttle valve, and the springs are, specifically, a closed side spring 111 and an open side spring 112. Thus, a stop opening degree of the throttle valve can be set at the middle stop opening degree (i.e., the throttle valve can be set at the middle position) so that a vehicle can be driven even in a case where the electronic throttle is broken and the electric motor 107 is turned off.
Here, the middle stop opening degree used in the middle opening position returning device is set at a position such that an air flow rate required for the emergency running can be achieved. Because target middle stop opening degrees are different for different vehicles, the electronic throttle is provided with an adjusting screw 115 for further adjusting the middle stop opening degree.
In the adjustment of the middle stop opening degree, for example, a closed side stop position of a spring hook 114, which is formed at a connection between the closed side spring 111 and the open side spring 112, is adjusted. The closed side stop position of the spring hook 114 is adjusted by advancing the adjusting screw 115 toward or away from the case 104 (see FIG. 6).
FIG. 7 shows a screw structure of the screw apparatus, in which the adjusting screw is screwed to the case as described above.
A conventional screw structure includes a case J1, which has a female threaded member J2, and an adjusting screw J3, which is screwed into the female threaded member J2. Specifically, in a case where the case J1 is formed using metal, the metallic case J1 is provided with the female threaded member J2. In another case where the case J1 is formed using resin, a metallic female threaded member is molded with the resin case J1.
However, in the conventional screw apparatus, because a machining process for the female thread J2 is required, a cost is increased. Also, in a case where chippings remain in the thread of the female threaded member J2, the chippings may enter into the case J1 while the apparatus is in assembly or in use. As a result, the gear mechanism included in the case J1 may not work properly.
Also, there has been disclosed a technology, in which the case does not require the machining process of the female thread because a self-tapping screw serves as the adjusting screw. In this technology, firstly, a resin case is provided with a through hole, and then the self-tapping screw, which is the adjusting screw, is screwed into the through hole (see Japanese Unexamined Patent Publication No. 2004-122511 corresponding to US Patent Publication No. 2004/0062595).
However, even in the technology disclosed in Japanese Unexamined Patent Publication No. 2004-122511 corresponding to US Patent Publication No. 2004/0062595, the chippings may be generated when the adjusting screw (the self-tapping screw) is screwed into the through hole. This is because a part of the case (e.g., an inner wall of the through hole) is cut by a male thread portion of the adjusting screw. Then, the chippings generated by the adjusting screw may enter into the case. Therefore, similar to the screw structure with a machined female threaded member, the chippings may enter into the case while the apparatus is in assembly or in use. As a result, the gear mechanism included in the case J1 may not work properly.